Je-Hyung Cho

Design of a Digital Offset Compensator Eliminating Transformer Magnetizing Current Offset of a Phase-Shift Full-Bridge Converter

A transformer magnetizing current offset for a phase-shift full-bridge (PSFB) converter is dealt with in this paper. A model of this current offset is derived and it is presented as a first-order system having a pole at a low frequency when the effects from the parasitic components and the switching transition are considered. A digital offset compensator eliminating this current offset is proposed and designed considering the interference in an output voltage regulation. The performances of the proposed compensator are verified by experiments with a 1.2-kW PSFB converter. The saturation of the transformer is prevented and the efficiency is improved by this compensator.

Implementation of digitally controlled phase shift full bridge converter for server power supply

The digital controller of the phase-shift full bridge converter for a server power supply is designed and implemented in this paper. Analog controllers are replaced by single MCU (micro controller unit) that not only manages the protection or the operational flow of a power supply but also controls the converter. Voltage mode controller is designed based on the z-domain small signal model considering effect of variation on output filter inductance. Moreover, gain scheduling is applied to adjust the controller gain according to load current. The current offset flowing through a magnetizing inductor is eliminated by adding PI compensator instead of inserting a blocking capacitor that may increase loss, cost, and volume. The saturation of a transformer and the conduction loss due to the current offset can be prevented. The performance of designed controller is verified by the experiments on 1.2kW phase-shift full bridge converter.

Start-Up Control to Prevent Overcurrent During Hot Swap in Paralleled DC–DC Converters

Paralleled power systems require a hot swap which indicates the replacement of a converter module without shutting down the whole system for easy maintenance, repair, and upgrade. A practical issue for the hot swap is an overcurrent toward a newly activating module due to inevitable nonuniformity among the paralleled modules, which causes the excessive current and thermal stresses and even leads to a device failure. In this paper, by analyzing the start-up operation of a converter module during the hot swap, the reason for the overcurrent is demonstrated. Then, based on this analysis, a method increasing the voltage reference exponentially is presented, which can contribute to reduce the overcurrent. However, the overcurrent still remains and causes a problem as the number of paralleled modules or difference in output voltage set points increases. Therefore, a control scheme adjusting the output voltage reference during the start-up is proposed to eliminate the overcurrent effectively. Experimental results from 1-kW paralleled phase-shifted full-bridge converters are shown to verify the proposed works.

A cold cathode fluorescent lamp driving circuit without a transformer for liquid crystal display backlight unit

A cold cathode fluorescent lamp driving circuit without a transformer for liquid crystal display backlight unit is proposed. Commonly, a CCFL driver has a transformer with high turn ratio to step up low input voltage to high sinusoidal lamp voltage. This high voltage and high turn ratio transformer has large size and high cost. The proposed CCFL driving circuit which is based on parallel-loaded resonant topology features simplicity, so it reduces complexity and cost of overall system. And the proposed circuit with asymmetric control has dimming capability by duty control. In this paper, the analyses of the proposed CCFL driving circuit and experimental results for BLU of 32-inch commercial LCD TV are presented.

Digital load share controller design of paralleled phase-shifted full-bridge converters referencing the highest current

For a high power demand and N+1 redundancy system, this paper presents a digital load share (LS) controller design and its implementation for paralleled phase-shifted full-bridge converters (PSFBC) used in distributed power systems. By adopting the digital control strategy, separately used ICs for both PSFBC and LS control functions in the analog system can be merged into a cost-effective digital controller. To compensate and stabilize PSFBC and LS loops with the direct digital design approaches, a small-signal model of the overall system is derived in the discrete-time domain successively. Then, the steady-state and dynamic load sharing performances are also investigated. Experimental results from two identical 1.2 kW paralleled PSFBC modules are presented to verify the proposed work.

Digital Lamp Balance System With Individual Lamp Current Control Capability for an LCD TV

A digital lamp balance system that equalizes lamp current of liquid crystal display (LCD) backlight is implemented in this letter. The proposed digital balance circuit consists of a balance switch which is in series with the lamp and a dc-blocking capacitor which also acts as a ballast capacitor for parallel lighting. The parasitic capacitance of the balance switch is used as parallel impedance which makes the lamp current decrease. Microcontroller unit controls each lamp current by changing the pulsewidth modulation duty ratio of a balance switch. The specified circuit, analysis, balance principles, and experimental results of the proposed system will be presented for a 46-in LCD TV.

Digital CCFL drive system using individual current modulation for LCD-TV

Current balance system using MCU (micro controller unit) which controls the individual lamp current is implemented in this paper. The system consists of a ballast capacitor for parallel lighting, an amplitude modulator switching between the on-state and the off-state for regulating the lamp current, a gate driver to shift a voltage level, and a current sensor to send a feedback signal to the MCU. Each lamp current is compared with a reference and appropriate gate signal is generated to balance the lamp current. The MCU controls lamp currents individually by changing the PWM duty cycle of a switch series connected with a lamp. An amplitude modulator using a junction capacitance as an off-state parallel impedance to control the lamp current is a switch having an anti-parallel body diode. Series connected ballast capacitor and off-state parallel capacitance play a role of a bidirectional current amplitude modulation with a switch regardless of the anti-parallel body diode. By employing only one switch for each lamp, the number of power supplies isolated from the system for driving gate signal can be minimized. The specified circuit configuration, analysis, balance principles, and experimental results of the proposed system will be presented for 46 inch LCD-TV.

A Hybrid PWM Resonant Converter Suitable for Wide Input Variation

A resonant converter suitable for applications having a wide input range is proposed. The proposed converter is a hybrid combination of a half bridge converter and a phase-shift full-bridge converter and they use one leg of switches in common. The secondary sides of two transformers are connected in series with a resonant capacitor in between which forms a resonant tank with the leakage inductances. By generating a controllable four-level voltage, the proposed converter can handle input voltage variation with reduced output filter size. The proposed converter achieves zero voltage switching easily along wide load range without noticeable increase in a circulating current and an effective duty loss. The analysis of the resonant converter and experimental results of 400W prototype is presented to verify the features of the converter.

A Three-Level PWM Resonant Converter with Wide ZVS Range employing Hybrid Structure

A hybrid structure three-level resonant converter suitable for wide input and load variation is proposed. A hybrid combination of a three-level converter controlled by phase-shift modulation and a half-bridge converter is presented. Since the voltage of each switch is one half of the input voltage, it has advantages of the choice and characteristics of switches. The ZVS operation of the converter is achieved by using the magnetizing current of the transformer. To verify the theoretical analysis, experimental results of the proposed converter are presented.